In print operations, liquid printing agents such as inks, fixers, primers and coatings may be applied to a substrate. In some examples, liquid print agents are expelled from the nozzles of a print head in ‘ink jet’ print operations. In one such technology, so called ‘bubble jet’ printing, print agent in a fluid cell is locally heated to cause formation of a vapour bubble. The resulting increase in pressure within the cell causes the ejection of a print agent droplet from a nozzle in the fluid cell.